High exposure to RF radiation (RFR) in rodents resulted in tumors in tissues surrounding nerves in the hearts of male rats, but not female rats or any mice, according to draft studies from the National Toxicology Program (NTP). Overall, there was little indication of health problems in mice related to RFR, an NTP statement said.

The exposure levels used in the studies were equal to and higher than the highest level permitted for local tissue exposure in U.S. cellphone emissions. Cellphones typically emit lower levels of RFR than the maximum level allowed.

NTP’s draft conclusions were released as two technical reports, one for rat studies and one for mouse studies. NTP will hold an external expert review of its complete findings from these rodent studies 26 – 28 March.

The incidence of tumors, called malignant schwannomas, observed in the heart increased in male rats as they were exposed to increasing levels of RFR beyond the allowable cellphone emissions. Researchers also noted increases in an unusual pattern of cardiomyopathy, or damage to heart tissue, in exposed male and female rats.

The reports also point out statistically significant increases in the number of rats and mice with tumors found in other organs at one or more of the exposure levels studied, including the brain, prostate gland, pituitary gland, adrenal gland, liver and pancreas. However, the researchers determined that these were equivocal findings, meaning it was unclear if any of these tumor increases were related to RFR.

"The levels and duration of exposure to RFR were much greater than what people experience with even the highest level of cellphone use and exposed the rodents' whole bodies. So, these findings should not be directly extrapolated to human cellphone usage," said John Bucher, Ph.D., NTP senior scientist. "We note, however, that the tumors we saw in these studies are similar to tumors previously reported in some studies of frequent cellphone users."

To conduct the studies, NTP built special chambers that exposed rats and mice to different levels of RFR for up to two years. Exposure levels ranged from 1.5 to 6 watts per kilogram (W/kg) in rats, and 2.5 to 10 W/kg in mice. The low power level for rats was equal to the highest level permitted for local tissue exposures to cellphone emissions. The animals were exposed for 10-minute on, 10-minute off increments, totaling just more than nine hours each day.

The studies used 2G and 3G frequencies and modulations still used in voice calls and texting in the United States. More recent 4G and 5G networks for streaming video and downloading attachments use different cellphone signal frequencies and modulations than NTP used in these studies.

The NTP studies also looked for a range of noncancer health effects in rats and mice, including changes in body weight, evidence of tissue damage from RFR-generated heating and genetic damage. Researchers saw lower body weights among newborn rats and their mothers, especially when exposed to high levels of RFR during pregnancy and lactation. Yet, these animals grew to normal size.

"These studies were complex and technically challenging, but they provide the most comprehensive assessment, to date, of health effects in rats and mice from exposure to RFR," said Bucher. "Cellphone technologies are constantly changing, and these findings provide valuable information to help guide future studies of cellphone safety."

The U.S. Food and Drug Administration (FDA) nominated cellphone RFR for study by NTP because of widespread use of cellphones. FDA and the FCC are jointly responsible for regulating wireless communications devices in the United States.

“Looking at the results in animals, the conclusions still require careful discussion, as our preliminary understanding of the NTP results is that the study found mostly equivocal, or ambiguous, evidence that whole body RF energy exposures given to rats or mice in the study actually caused cancer in these animals,” said Dr. Jeffrey Shuren, director of the FDA's Center for Devices and Radiological Health. “There are additional unusual findings from the study, such as the exposed rats living longer than the control group rats, that we are assessing to understand how that may be relevant to the results.”

A March peer review of the study is an important and crucial step in scientific research to assure the integrity and quality of the data and the conclusions that can be drawn from it, he said.

“I want to underscore that based on our ongoing evaluation of this issue and taking into account all available scientific evidence we have received, we have not found sufficient evidence that there are adverse health effects in humans caused by exposures at or under the current radiofrequency energy exposure limits,” Shuren said. “Even with frequent daily use by the vast majority of adults, we have not seen an increase in events like brain tumors. Based on this current information, we believe the current safety limits for cell phones are acceptable for protecting the public health.”

Public comment can be provided through the NTP’s Federal Register notice. The draft technical reports are here.

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